Turbine compressors often utilize adjustable guide vanes in the inlet of the turbine to control air flow and pressure over a range of operation. The guide vanes are typically arranged in a row in the stationary (non-rotating) part of the compressor casing, and in some cases, 40 to 60 or more vanes are used on each turbine. The vane blade stem ends typically extend through a compressor casing, and may be attached to a linkage that simultaneously turns each individual vane blade. For example, a “uni-center ring” may be utilized to turn each individual blade as the ring is rotated circumferentially around the compressor inlet casing. Depending on whether one or more rows of these vanes are to be controlled, linkage elements may be ganged together to control the rows of vanes in unison.
To move the linkage, and in turn, adjust the guide vanes, a servo system including a hydraulic actuator is typically employed. For example, the position of the hydraulic actuator may be monitored and fed back to a controller in the servo system using transducers such as resolvers, linear variable differential transformers (LVDTs) or linear variable differential reluctance (LVDR) devices. One of the complications with such a system is that there may be a complex and non-linear relationship between the transducer measurement and the actual angle of the vane blades due not only to geometric and rotational transformations, but also to manufacturing tolerances and wear in the linkage. A lack of precision in the positioning of the guide vanes and/or variable stator vanes can result in a corresponding lack of precision in the control of flow through the machine, possibly resulting in a loss of output or efficiency or both.